Measurements of NO2 Concentration with Wide Dynamic Range Based on Laser Absorption Spectroscopy Using a Multi-Pass Optical Cell (- Effects of Ambient Temperature Change at the Optical Cell -)

摘要

This paper discusses the precise measurement of NO2 concentrations in a sampled gas mixture using laser absorption spectroscopy. Pressurized NOVNi gas with known NO2 concentrations were used as sample gas mixtures. A frequency doubled Nd: YAG laser and mid-infrared (MIR) coherent light, using difference frequency generation techniques, were used to examine optical absorption by NO2. A relatively small change in temperature (AT ~ 3 K) at the optical multipass cell was found to vary the optical depth, particularly in a high concentration of NO2 mixture. This change is likely caused by a non-equilibrium dimerization effect between NO2 and N2O4 induced by the transient temperature non-uniformity of the gas mixture. Theoretical calculations and spectroseopic analysis suggest that careful consideration needs to be given to the stabilization of optical cell temperature, which is required to calibrate the laser absorption spectroseopic system for precise measurements of NO2 concentration.